1. Field of the Invention
This invention relates to a sintered oil-impregnated bearing having a smoothed surface and a porous surface formed on the inner peripheral surface of the bearing.
This application has subject matter which is related to application Ser. No. 584,158, filed Sep. 18, 1990, entitled Oil Impregnated Sintered Bearing, assigned to the same assignee.
2. Background Prior Art
A conventional sintered oil-impregnated bearing is prepared typically by following the process schematically illustrated in FIG. 11 of the accompanying drawings and including the steps of weighing the ingredients of the compound, mixing, molding and sintering the compound. Such a bearing has one, or more than one, porous surface formed by oil-filled pores on the inner peripheral surface of the bearing. When a bearing of this type is used with a rotary shaft 19 of a capstan motor 18 of an audio- or video recording apparatus as illustrated in FIG. 12 and the rotary shaft 19 is subjected to lateral pressure as shown in FIG. 13, a large portion of the oil contained within the bearing can flow out, based on the principle schematically illustrated in FIG. 14, so that adjoining metal components can cause friction that can result in abnormal abrasion (scar a) and seizure of the metal components as the oil film protecting the bearing is moved away. In order to avoid this problem, conventional sintered oil-impregnated bearings are subjected to an aging operation or a cutting operation using a sizing margin as described in Japanese Patent (Jikkai) No. 50-101735.
With the lubricating system of a conventional sintered oil-impregnated bearing as schematically illustrated in FIG. 14, oil can escape, to a large extent, from the pores of the porous surface A of the bearing 20 when the motor is started. As a result, a phenomenon of boundary lubrication takes place, causing a large shaft loss current value to occur. Thereafter, the frictional surface of the motor is plastically deformed by the friction with the rotary shaft 19 so that the porous surface is smoothed away and a high oil pressure is generated to bring forth a phenomenon of fluid lubrication as indicated by a curved arrow B. This, in turn, causes a phenomenon of aging where the porous surface is smoothed out to reduce the shaft loss current value until a stable level is reached. The aging operation is conducted to attain the stable level without problem.
FIG. 15 shows a plan view of a conventional molded and sintered oil-impregnated bearing 21, where the broken line indicates the inner peripheral surface 22 after sizing and the solid line indicates the inner peripheral surface 23 with a sizing margin of 50 .mu.m. FIG. 16 shows the change with time of the shaft loss current value.
Both the aging operation as described above and the use of a sizing margin as disclosed in the above cited Japanese Patent document are accompanied by the following drawbacks.
1) Where the aging operation is involved:
a) Since the motor has to be driven for 0.5 to 4 hours in a variable speed mode with a constant side pressure for the aging operation in an aging apparatus, the cost for the required man-hours, equipment and space can be enormous.
b) While every aging apparatus is specifically designed for a particular purpose (in view of the variety of customers' requirements in terms of side pressure) and can accommodate only a specific direction of side pressure (unable to cover a certain angular width), it cannot necessarily meet the requirements of a specific customer and troubles can result.
Further, the accuracy with which a particular machine installed by the supplier operates does not necessarily match the accuracy with which the aging apparatus operates and, therefore, there can be a discrepancy in the level of accuracy, causing operational difficulties.
2) With the technique disclosed in the above cited Japanese Patent Document, an axial half of the inner peripheral surface of the bearing of the rotary shaft that actually bears the load is smoothed out to prevent any run-off of oil film as indicated by a dotted semicircular line in the drawing, while the other half that does not bear any load remains porous to allow the oil to flow out.
However, the bore of the molded bearing does not present an exactly circular cross section but has a semi-circular sizing margin and the outer periphery shows a circular cross section which is not necessarily concentric with that of the inner periphery. These facts can constitute drawbacks for this technique. Besides, since the sizing margin for smoothing has large dimensions, the metal metallurgic structure of the bearing can be damaged or become brittle to degrade its surface precision.
Moreover, since the smoothed area of the bearing shows an angular width of more than 90.degree. and constitutes nearly half of the inner periphery to reduce the remaining porous area from which oil can emerge, it presents poor oil circulation and consequently a high or fluctuated friction torque, particularly, when the motor rotates at a high speed at low temperature.
In view of the above mentioned problems of the prior art, it is therefore an object of the present invention to provide a sintered oil-impregnated bearing having an exactly circular bore cross section that does not require any aging operation and can be manufactured without significantly altering the existing manufacturing facilities by forming a plurality of evenly extending smoothed areas on the inner peripheral surface of the bearing, the smoothed surfaces having a constant angular width of 10.degree. to 90.degree., the remaining surface areas between any adjoining two of said smoothed areas being kept porous.